Genetic Analyses of Male-Fertility Restoration in Wheat. II. Isolation, Penetrance, and Expressivity of Rf Genes

A previous paper reported that the Rf1 and Rf4 genes (controlling male fertility) were located on chromosomes 1A and 6B, respectively, of an alloplasmic Triticum aestivum L. R-line (R113) with T. timopheevii Zhuk. cytoplasm. This paper reports the development of wheat lines with individual Rf genes from RI13 and the relative penetrance and expressivity of the Rfl and Rf4 genes. Male-sterile ‘Chris’ (mst-Chris) monosomic 1A and monosomic 6B (with T. timopheevii cytoplasm) were crossed with R113; and cytologically identified monosomic F₁'s were crossed as females with euplasmic Chris pollen. The number of malefertile and sterile disomic and monosomic segregants and the mean number of seed/spike of the fertile segregants were compared between and within the two sets of F₁, F₂, BCF₁, BCS₁, BCS₂, and testcross progenies; i.e., mst-Chris ✕ BCF₁, mst-Chris ✕ BCS₁ and mst-Chris ✕ BCS₂ progenies. In the BCF₁ monosomic IA and monosomic 6B progenies, most disomics were fertile because they retained the Rfl gene on 1A or Rf4 gene on 6B, respectively. In contrast, the fertile monosomic plants in BCF₁ monosomic IA progeny retained the Rf4 gene on 6B; but the Rfl gene on 1A was deleted. Similarly, fertile monosomic plants in BCF₁ monosomic 6B progeny retained the Rfl gene on 1A; but the Rf4 gene on 6B was deleted. A substantially higher proportion (indicating greater penetrance) of BCF₁ fertile monosomics with Rf4 were obtained than BCF₁ fertile monosomics with Rfl. Also, fertile monosomic 1A with Rf4 had a greater number of seed/spike (indicating greater expressivity) than fertile monosomic 6B with Rfl. However, the expressivity of the Rf genes was reversed in the euploid testcross progenies. Thus, the expression of Rf4 was affected to a lesser extent by the deficiency of one homologue of 1A than the expression of Rf1 was affected by the deficiency of one homologue of 6B.